Although there are a variety of uses for the device of the present invention, specific reference to the use of the present invention in a milking operation will be emphasized. When milking cows, goats or other milking animals with automated milking apparatus, it is important to determine the end of the milking period. Generally, the end of the milking period is determined by observing the udder being milked, the condition of the milk obtained or the quantity of milk as it flows in a milk line which has been provided with a transparent portion. For example, U.S. Pat. No. 3,007,436 to Seaborne discloses a milk flow indicator which comprises a sight glass for visual indication of the end of the milking period. U.S. Pat. No. 2,697,944 to Wenham also discloses a liquid flow indicator for a milking machine which indicates visually the state of milk flow through the indicator. U.S. Pat. No. 2,513,627 to Dinesen discloses a flow indicator for a milking machine having a transparent portion making it possible for the operator to detect milk flow at all times.
Other methods of determining the end of the milking period are also described in the prior art. U.S. Pat. No. 3,991,716 to Reisgies discloses an apparatus for sensing milk flow from an animal being milked using pneumatic controls without the use of electrical circuitry. U.S. Pat. No. 3,786,762 to Abrahamson et al. discloses a milk flow sensing valve unit comprising a float chamber housing a float member which holds the valve open when the milk is present to permit milk discharge. When the milk flow falls below a certain rate, the float lowers and closes the valve shutting off the vacuum supply causing the teat cups to fall away. U.S. Pat. Nos. 3,973,520 and 4,022,158 to Flocchini disclose a float detector comprising a cylindrical flow tube with a metal float. The milk in the tube causes the float to rise. As the level of milk drops, the float moves toward a metal detector at the bottom of the tube. When the float contacts the detector, a circuit is activated sensing the interruption of milk flow. If milk flow is then resumed, the float moves away from the detector and the detection of resumption of milk flow is resumed. Another patent which uses electrical sensors to determine liquid flow is U.S. Pat. No. 4,433,577 to Khurgin et al. U.S. Pat. No. 4,348,984 to Brayer discloses a flow system controller which automatically stops the milking operation when the flow of milk is decreased. The milk flow sensor employs a pair of electrodes to sense the flow of milk. The flow of milk causes an electric current to flow between the electrodes. When the milk flow drops off, an output signal stops the pulsator and produces a warning light. U.S. Pat. No. 4,306,517 to Nakamura discloses a milk flow sensor which signals the end of the milking operation. A pair of electrodes are used for detecting the quantity of milk flowing through the sensor. An alarm device signals the end of milking.
U.S. Pat. No. 4,253,421 to Slater et al. discloses a milk flow system connected to a stanchion which detects the flow of milk by means of a counterweight system. When milk is in the sensor, flow communication is opened allowing the vacuum system to operate. When the milk flow drops, the counterweight swings closing off the vacuum which automatically removes the teat claws.